Hi, Tom. Thanks for your implication!
TomGeorge:
How big is the 9V battery?
It's one of those common rechargeable 250 mAh 9V block.
TomGeorge:
What voltage are the servos rated at?
Where do they get their power from?
What rating are the servos, link to specs/data?
Both are Vigor VS-2 servos. They have an operating voltage of 4.8-6 volts.
You can check the specs here:
TomGeorge:
What do the servos operate?
The throttle servo operates the engine's throttle lever. It's so light, it literally dangles about when it's not connected to the servo's arm.
The steering servo controls the front steering wheels. They're made out of plastic, so they ain't too heavy.
TomGeorge:
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Sure thing: Imgur: The magic of the Internet
TomGeorge:
Have you measured your battery volts while the fault occurs?
I couldn't really, since I was rather freaked out by the engine revving like hell.
It never happened before, and I was using exactly the same devices and servos. Could a capacitor keep the power up for the Arduino during a potential power loss? Thanks.
P.S. In the sketch I didn't connect the HC-05 to the Arduino other than the 5v and the GND, since I reckon they're not affecting the voltage.